Officers were called to the venue on Haverstock Hill, NW3, on Sunday, 27 December after receiving reports of multiple people entering the venue.
Officers attended and inside the basement found 10 men and 10 woman inside. All attendees were issued a Fixed Penalty Notice (FPN) for breaching the tier 4 rules.
Inspector Alex Hope-Rollinson said: “To date, over 70,000 people in the UK have been lost to coronavirus.
“Breaches of coronavirus regulations such as these are selfish, thoughtless and ultimately put the safety and health of our community at serious risk.
“I implore individuals to make themselves familiar with the Tier 4 rules and absolutely avoid attending any gatherings which are in breach of those rule – it is vital that we work together to avoid overwhelming our NHS in the coming months.”